[clang][c++20] Differing behaviour vs msvc and gcc when mixing CRTP and concepts

[octurion]

I was implementing an idea from the following blog post and the followup discussion on Reddit so as to enforce that any CRTP derived class would implement the interfaces the CRTP base class relies on.

The code is as follows: testcase.txt

However, clang seems to reject this specific piece of code (https://godbolt.org/z/9KGhTPncW).

What is more surprising is that it appears that both:

• gcc (https://godbolt.org/z/aWqre6rf5)
• and msvc (https://godbolt.org/z/fjbKPqr6j)

Accept this piece of code.

This means that at least one compiler is incorrect or (highly unlikely, but you never know) the C++ standard is ambiguous in that specific case.

[llvmbot]

@llvm/issue-subscribers-c-20

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[usx95]

This looks related to conditionally trivial special member functions and deferred concept instantiation.
Notice that clang accepts if the requires clause is on member function crtp_method (https://godbolt.org/z/seGE499ds)
CC: @erichkeane @royjacobson
Looks like a known issue: #44178 (comment)